Method of making α-silicon nitride powder

ABSTRACT

α-Silicon nitride powder which is used as a raw material for the preparation of high strength silicon nitride with additives such as magnesia and yttrium oxide, and other sintered materials suitable for high temperatures gas turbine engine components and the like, is prepared by heating a powdered mixture of silica, carbon and at least one component selected from the group consisting of silicon nitride, silicon carbide and silicon oxynitride in a nitrogen containing atmosphere and then optionally subjecting the material to a heat treatment in an oxidizing atmosphere for decarbonization of said material as required.

This is a continuation of application Ser. No. 878,452 filed 2/16/78,now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for preparing α-Si₃ N₄, thatis, the alpha form of silicon nitride, and more particularly to α-Si₃ N₄powder of high quality which is obtained in a high yield and aconsistent yield.

2. Description of the Prior Art

It is known that sintered silicon nitride-yttrium oxide or magnesiumoxide (Si₃ N₄ -Y₂ O₃ or Si₃ N₄ -MgO) materials possess excellentmechanical strength and heat resistance, and therefore have been used inhigh temperature gas turbine engines. However, when the conventional Si₃N₄ sintered products are used in practice as materials which aresubjected to high temperatures and high stresses, their physical andchemical stabilities and reliability at high temperatures are absolutelyessential requirements. Their thermal and mechanical properties, whichare particularly important factors, are greatly affected by the natureof the starting materials and the quantities of impurities which thesematerials contain. Moreover, with regard to the silicon nitride it isdesirable that it should contain as much α-Si₃ N₄ powder as possible.Especially desired is a finely divided α-Si₃ N₄ powder for use insintering materials.

In the past, Si₃ N₄ powder has been synthesized by the followingmethods.

(1) 3Si+2N₂ →Si₃ N₄

(2) A vapor phase reaction in which silicon tetrachloride or silane isreacted with ammonia as starting materials 3SiCl₄ +4NH₃ →Si₃ N₄ +12 HCl,and the like;

(3) A method of nitridizing SiO₂ obtained by reducing silica (SiO₂) withcarbon in the following stoichiometric ratio 3SiO₂ +6C+2N₂ →Si₃ N₄ +6CO,and the like.

In the case of method (1), the nitridization of Si is an exothermicreaction, and therefore the process must be carefully conducted so as tocarefully control the generation of heat. For example, the Si which iscommercially selected for the reaction is comparatively coarse-grainedpowder, and therefore, fine grinding is generally conducted afternitridization. Therefore, the admixture of impurities into the productduring the grinding process is unavoidable, and although there is noobjection to the use of this material for refractory materials ingeneral, such as firebricks, it is not suitable for high temperature gasturbines.

The process of reaction (2) yields a product which is suitable, forinstance, for the surface coating of semiconductor elements and thelike, but it cannot be regarded as suitable for the mass production ofinorganic refractory materials.

In the case of reaction (3) thoroughly purified SiO₂ powder and carbon(C) powder must be used as starting materials, and there is also thedisadvantage that the product produced by reacting stoichiometricquantities of SiO₂ and C comprises a mixed system of α-Si₃ N₄, β-Si₃ N₄(beta form of silicon nitride), silicon oxynitride (Si₂ ON₂), siliconcarbide (SiC) and the like. Moreover, the yield of α-Si₃ N₄ is low. Inother words, this reaction system has the advantage that the reactionprocedure is relatively easy, but the yield of α-Si₃ N₄ product is low,and therefore the method is not preferred in practice.

A need, therefore, continues to exist for a method by which high qualityα-silicon nitride can be manufactured in high and consistent yield.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide a methodof manufacturing high quality α-silicon nitride in high and consistentyield.

Another object of the invention is to provide a high quality α-siliconnitride.

Still another object of the invention is to provide a high qualityα-silicon nitride powder which is suitable for use as refractorymaterial in high temperature and high stress environments.

Briefly, these objects and other objects of the present invention ashereinafter will become more readily apparent can be attained byproviding a method of manufacturing α-silicon nitride which comprisesthe step of heating a powdered mixture of 1 wt. part silica, 0.4-4 wt.parts carbon and 0.005-1.0 wt. part of at least one component selectedfrom the group consisting of silicon nitride, silicon carbide andsilicon oxynitride at from 1350° C. to 1550° C., preferably from 1400°C. to 1500° C. in a non-oxidizing atmosphere containing at least one ofnitrogen or ammonia, such that reduction and nitridization reactionsoccur which produce silicon nitride, and optionally heating said siliconnitride at from 600° C. to 800° C. in an oxidizing atmosphere to effectdecarbonization after said nitridization.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The essential feature of the discovery of the present invention is thatif in the reduction and nitridization of silica (SiO₂), the quantity ofcarbon (C) which is used is a quantity equal to or somewhat in excess ofthe amount needed to reduce SiO₂ and if a specified quantity of at leastone component selected from the group consisting of silicon nitride,silicon carbide and silicon oxynitride is also present, and thenitridizing reaction, and as required, a heat treatment under anoxidizing atmosphere are conducted at specified temperatures, then α-Si₃N₄, that is, the alpha form of silicon nitride, of high quality, andwhich is extremely fine-grained, can be produced in high yield.Furthermore, the yield which is obtained is consistent even ifimpurities are contained in the starting material, such as Fe or Fecompounds in the carbon powder.

The present invention involves a method of manufacturing α-siliconnitride (α-Si₃ N₄) powder, characterized in that a mixture of powderedreactants, in which the ratio proportions of the reactants by weight are1 part silica (SiO₂) powder, 0.4 to 4 parts carbon (C) powder and 0.005to 1.0 parts of at least one component selected from the groupconsisting of silicon nitride, silicon carbide and silicon oxynitride(Si₃ N₄, SiC and/or Si₂ ON₂) powder, is heated and fired at 1350° to1550° C., under a nonoxidizing atmosphere containing at least one ofnitrogen or ammonia, wherein reduction and nitridization reactions takeplace and silicon nitride is produced. Thereafter, the product obtainedis optionally subjected to a further heat treatment in an oxidizingatmosphere such as air, preferably at 600° to 800° C. as required. Inthe invention, the term carbon includes higher hydrocarbons. Suitablesources of carbon for the method of the invention include carbon powderssuch as carbon black, and hydrocarbon materials such as naphthalene,anthracene, and the like.

In the silica-carbon-silicon nitride, silicon carbide and/or siliconoxynitride (SiO₂ -C-Si₃ N₄, SiC and/or Si₂ ON₂) mixture used as thestarting material in the present process, the preferred SiO₂ :C:Si₃ N₄,SiC and/or Si₂ ON₂ weight ratio is selected as 1:0.4 to 4:0.005 to 1.0for the following reasons. If less than 0.4 part of C is used to 1 partof SiO₂, then a large quantity of Si₂ ON₂ is formed and the quantity ofthe α-silicon nitride produced is small and some unreacted SiO₂ remains.If necessary, the SiO₂ containing silicon nitride may be prepared. Onthe other hand, if more than 4 parts of C are used per 1 part of SiO₂,then β-Si₃ N₄, that is, the beta form of silicon nitride, is producedand as a result the purity and the yield of the α-silicon nitride arereduced. Furthermore, if less than 0.005 part of Si₃ N₄, SiC and/or Si₂ON₂ is used per part of SiO₂, the effect upon increasing the yield of α-silicon nitride is inadequate. On the other hand, if more than 1.0 partof Si₃ N₄, SiC, and/or Si₂ ON₂ is used per part of SiO₂, the precessbecomes too uneconomical. In this case large amounts of added powdersare contained in the products and the preferred silicon nitride powderis not necessarily obtained.

The SiO₂, C and Si₃ N₄, SiC and/or Si₂ ON₂ starting materials preferablyare of high purity of at least 99%. SiO₂ may include silica formed byheat treatment. Moreover, concerning grain sizes, SiO₂ and C preferablyhave an average grain diameter not greater than 1 μm and Si₃ N₄, SiCand/or Si₂ ON₂ have an average grain diameter not greater than 2 μmpreferably 0.5 μm. Small grain sizes of starting materials arepreferable in the process. When silicon nitride is used as startingmaterial for manufacturing α-Si₃ N₄ powder, it is preferably relativelypure in comparison with the SiC or Si₂ ON₂ used the process. The siliconnitride starting material may contain metallic silicon as an impurityand may include amorphous or non-crystalline silicon nitride.

When the starting materials are SiO₂ -C-Si₃ N₄, the Si₃ N₄ may be eitherthe alpha or the beta form, but the alpha form is preferred. It is alsoacceptable to use Si₃ N₄ which contains minor amounts of other elementssuch as Aluminum or Oxygen in solid solution.

In the heating and firing of the SiO₂ -C-Si₃ N₄, SiC and/or Si₂ ON₂mixture in the process of the present invention, the atmosphere over thereactants can be N₂, NH₃, N₂ and hydrogen (H₂), N₂ and an inert gas suchas Ar, He or the like, but the main reaction gas constituent must be atleast one of N₂ or NH₃. The reason for this is that it has beenconfirmed experimentally that at least one of these gaseous materials isnecessary to realize the desired great effect on the production ofhighly pure α-Si₃ N₄. The heating and firing temperatures used in theprocess of the invention under this atmosphere in which the mainreaction gas is N₂ and/or NH₃ is selected within the range from 1350° to1550° C. The reason for this is that if the temperature employed is lessthan 1350° C., Si₃ N₄ is not formed readily. If the temperature exceedsthe upper limit, excessive formation of SiC occurs, and the requiredα-Si₃ N₄ powder suitable for materials which are to be used under hightemperatures and high stresses cannot be obtained in adequate yield andpurity.

Also, after the silicon nitride product is heated and fired in anatmosphere in which the main reaction gas is N₂ or the like, a heattreatment under an oxidizing atmosphere such as air is advantageouslyconducted for the purpose of removing the residual carbon. Thetemperature of this treatment is selected within the range of 600° to800° C. Temperatures in excess of 800° C. tend to result in oxidation ofthe Si₃ N₄ and decreased yields, while temperatures below 600° C. areinadequate to efficiently remove carbon.

If, as described above, the reduction and nitridization of SiO₂ by thepresent invention is employed, in which an excess of carbon well abovethe stoichiometric amount is used, and a specific quantity of Si₃ N₄ isalso present, then the reduction of the SiO₂ is substantially promoted.Also, the added Si₃ N₄ serves as nuclei for smooth crystal growth ofsubsequently formed product and α-Si₃ N₄ powder of high quality,containing a large quantity of α-Si₃ N₄, is obtained in good yield.

When the method of the present invention is used, α-Si₃ N₄ powdersuitable for making silicon nitride sintered materials which arerequired to withstand high temperatures and high stresses, is easilyobtained. The reason for this method is believed to be as follows. Theprimary reaction which occurs is the reduction of silica by carbon, SiO₂+C→SiO+CO. This reaction is a solid phase reaction, and when the C/SiO₂ratio is high, the reaction becomes relatively rapid and the SiO whichis produced reacts with the N₂ or NH₃ more easily. In this reaction theSiO and the N₂ or NH₃ may be present in the vapor state, and thereforeit may be said that the proportion of carbon that is present governs thereduction and nitridization reactions of the SiO. In this instance, ifthe quantity of carbon is lower than the stoichiometric amount, Si₂ ON₂is formed, and the conversion of Si₂ ON₂ to α-Si₃ N₄ becomes extremelydifficult. However, as described above, the quantity of carbon isgreatly in excess compared to the stoichiometric amount, and it appearsthat for this reason the formation of Si₂ ON₂ is inhibited and α-Si₃ N₄is easily formed.

The presence of a large excess of carbon gives rise to the smoothproduction of α-Si₃ N₄. However, on the other hand, the presence ofcarbon may also result in the admixture of other impurities in theproduct which were present in the C powder used as a starting material.The α-Si₃ N₄ content of the product might ordinarily become relativelylow as a result. But in an embodiment of the present invention, aspecified quantity of Si₃ N₄ powder is also present in the reaction. Inthe production of Si₃ N₄ by an oxide-reduction reaction, theaforementioned SiO, N₂, NH₃, and the like are present in the vaporstate. Once Si₃ N₄ in the solid state is formed, it facilitates furthergrowth thereafter and influences the reaction speed and yield ratio.However, in the present invention, the Si₃ N₄ powder which is addedbeforehand operates as nuclei for further growth and formation of solidSi₃ N₄. Moreover, the amount of SiO vapor is reduced as a consequence ofcrystal growth of Si₃ N₄, which contributes significantly to theimproved purity of the Si₃ N₄ produced. Further, even if impurities arepresent in the reaction system, for example from Fe compounds present inthe carbon powder, α-Si₃ N₄ is still obtained in consistently highyield.

Thus, in the present invention an α-Si₃ N₄ powder is obtained which isof high quality and which contains a high content of α-Si₃ N₄. Moreover,the nitride contains only small quantities of SiC and other impurities.Thus, the method of the present invention is suitable for making Si₃ N₄powder which is suitable as a raw material for the preparation ofsintered structural materials which are required to withstand hightemperature and high stresses, for instance, for gas turbine enginecomponents.

Having generally described the invention, a more complete understandingcan be obtained by reference to certain specific examples, which areincluded for purposes of illustration only and are not intended to belimiting unless otherwise specified.

EXAMPLE 1

A SiO₂ powder having an average grain diameter of 13 mμm, C (carbonblack) powder having an average grain diameter of 29 mμm and Si₃ N₄powder having an average grain diameter of 1.0 μm were mixed in theproportions (parts by weight) shown in Table 1 to form 17 samples ofpowdered materials (including the reference examples).

The powdered mixtures were heated and fired at 1350° to 1550° C. for 2to 5 hours under a N₂, N₂ -H₂, N₂ -Ar or NH₃ atmosphere, and thensubjected to a heat treatment under an air atmosphere at 700° C. for 8hours, and Si₃ N₄ containing powders were thus obtained.

For each of the Si₃ N₄ containing powders thus obtained, the averageparticle size(S), the nitrogen content (weight %), the α-Si₃ N₄ content(weight %), which was confirmed by X-ray diffraction patterns, the SiCcontent (weight %) and the quantities of Si₃ N₄ and other metallicimpurities (weight %) were determined in each case and the results areall shown in Table 1. In Table 1, specimens 1 to 13 representembodiments of the present invention, and specimens a to d representreference examples.

EXAMPLES 2-3

An alternative embodiment was produced by replacing the Si₃ N₄ powder ofExample 1 as starting material by SiC powder having a particle size of0.8 μm, and yet another embodiment by replacing the Si₃ N₄ powder by Si₂ON₂ powder having a particle size of 1.5 μm. These were treated underthe same conditions and using the same volumes of materials as Nos. 1 to4 in Table 1. A high quality α-Si₃ N₄ powder with about 95% α-Si₃ N₄content was produced in each case.

                                      TABLE 1                                     __________________________________________________________________________    Composition   Reaction Conditions             Impuri-                         (weight ratio)                                                                              Temp.                                                                             T  Atmos-                                                                             Characteristics of Produced Powder                                                                ties                            materials                                                                          SiO.sub.2                                                                        C Si.sub.3 N.sub.4                                                                  (°C.)                                                                      (hr)                                                                             phere                                                                              S (μm)                                                                         N (%)                                                                             α-Si.sub.3 N.sub.4                                                             SiC (%)                                                                            (%)                             __________________________________________________________________________    1    1  2 0.1 1400                                                                              5  N.sub.2                                                                            1.2 37.7                                                                              95     0.3  0.09                            2    1  2 0.01                                                                              1400                                                                              5  N.sub.2                                                                            1.1 35.1                                                                              96     0.3  0.09                            3    1  2 0.005                                                                             1400                                                                              5  N.sub.2                                                                            1.7 34.2                                                                              94     0.3  0.09                            4    1  2 1.0 1400                                                                              5  N.sub.2                                                                            1.4 37.0                                                                              90     0.41 0.12                            5    1  4 0.1 1400                                                                              5  N.sub.2                                                                            1.1 37.5                                                                              96     0.22 0.13                            6    1  0.4                                                                             0.1 1400                                                                              5  N.sub.2                                                                            1.1 36.0                                                                              95     0.2  0.06                            7    1  2 0.1 1380                                                                              5  N.sub.2                                                                            1.2 36.1                                                                              95     0.28 0.08                            8    1  2 0.1 1450                                                                              5  N.sub.2                                                                            1.5 37.9                                                                              94     0.32 0.11                            9    1  2 0.1 1400                                                                              5  N.sub.2 + H.sub.2                                                                  1.1 35.7                                                                              96     0.26 0.10                            10   1  2 0.1 1400                                                                              5  NH.sub.3                                                                           1.2 38.0                                                                              95     0.28 0.10                            11   1  2 0.1 1400                                                                              5  N.sub.2 + Ar                                                                       1.2 36.3                                                                              94     0.28 0.08                            12   1  2 0.1 1480                                                                              2  N.sub.2                                                                            1.2 37.5                                                                              96     0.29 0.10                            13   1  0.4                                                                             0.1 1510                                                                              3  N.sub.2                                                                            1.0 37.5                                                                              92     0.5  0.08                            a    1  2 --  1400                                                                              5  N.sub.2                                                                            1.7 14.2                                                                              90     0.1  0.11                            b    1  4 --  1400                                                                              5  N.sub.2                                                                            1.5 21.2                                                                              91     0.25 0.17                            c    1  0.4                                                                             --  1400                                                                              5  N.sub.2                                                                            1.5 9.2 90     0.22 0.15                            d    -- --                                                                              1.0 1400                                                                              5  N.sub.2                                                                            1.0 38.0                                                                              70     --   0.01                            __________________________________________________________________________     INDEX OF TABLE 1                                                              SiO.sub.2 = silica powder                                                     C = carbon powder                                                             Si.sub.3 N.sub.4 = silicon nitride powder                                     temp. = temperature during reaction treatment (centigrade)                    T = time during reaction treatment (hours)                                    S = average grain diameter (micrometer)                                       N = nitrogen content (weight percent)                                         α-Si.sub.3 N.sub.4 = α-Si.sub.3 N.sub.4 content (weight           percent)                                                                      SiC = SiC content (weight percent)                                            impurities = other metallic impurities (weight percent)                  

Having fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the inventionas set forth herein.

What is claimed as new and intended to be secured by Letters Patentis:
 1. A method of manufacturing α-silicon nitride which comprises thesteps of:(a) premixing 1 wt. part silica powder, 0.4-4 wt. parts carbonpowder and 0.005-1.0 wt. part of at least one powder component selectedfrom the group consisting of silicon nitride, silicon carbide andsilicon oxynitride; and (b) heating said mixture at from 1350° C. to1550° C. in a non-oxidizing atmosphere containing at least one ofnitrogen or ammonia, for a time sufficient to effect the formation ofα-silicon nitride.
 2. The method of claim 1, which further comprises thestep of heating said α-silicon nitride at from 600° C. to 800° C. in anoxidizing atmosphere to effect decarbonization.
 3. The method of claim1, wherein said component is silicon nitride.
 4. The method of claim 3,wherein said silicon nitride is α-silicon nitride.
 5. The method ofclaim 1, wherein said non-oxidizing atmosphere is N₂ gas.
 6. The methodof claim 1, wherein said non-oxidizing atmosphere is NH₃ gas.
 7. Themethod of claim 1, wherein said non-oxidizing atmosphere is a mixture ofN₂ and H₂ gas.
 8. The method of claim 1, wherein said non-oxidizingatmosphere is a mixture of N₂ and Ar gas.
 9. The method of claim 2,wherein said oxidizing atmosphere is air.
 10. The method of claim 1,wherein said component is silicon carbide.
 11. The method of claim 1,wherein said component is silicon oxynitride.
 12. The method of claim 1,wherein said silica and said carbon have an average grain diameter notgreater than 1 μm.
 13. The method of claim 1, wherein said component hasan average grain diameter not greater than 2 μm.
 14. The method of claim13, wherein said component has an average grain diameter not greaterthan 0.5 μm.
 15. The method of claim 1, wherein said component has apurity of at least 99%.